When President Trump tweeted the other day that "I have the absolute right to PARDON myself", it started the engine on a furious debate.
Since so much of the 2016 Presidential election reflected interest in the Supreme Court and who would replace Justice Antonin Scalia, let's try an originalist interpretation of the type Scalia championed: Pardon, by the original definition of the word, is a thing to be given ("donare" -- "to give" -- being the root of the latter part of the word). According to an originalist interpretation, then, the word as used by the Founders represented something to be given.
It's not difficult to understand that you cannot give something to yourself; you can only take it. Consequently, a pardon could not be taken by a President for him- or herself. Pardon must be given by someone who was offended by the action. In the case of the Presidential pardon, the President stands in as the offended party on behalf of the American people -- so it would be again impossible by that definition to be both the party offended by the crime and the party who committed the crime.
Pardons must be seen as part of a three-legged stool, along with the prohibitions on ex post facto laws (Article I, Section 10 of the Constitution) and the prohibition on double jeopardy (the Fifth Amendment).
Taken together, those three components make it clear that in the United States, what is legal is what the law says at the time of the act. We don't go back and retroactively punish people for things we decide later to make illegal, nor do we punish people for acts for which they have already been forgiven.
If a President could self-pardon, then no remedy would exist to ensure their prosecution for illegal acts -- unless we were to suspend the limits on ex post facto or on double jeopardy. And, not just from an originalist standpoint but from a practical one, it would be a titanic mistake to allow the rules to shift according to the occupant of the Oval Office at the time: We want neither lawlessness now, nor arbitrary persecution (and prosecution) of people after they've left office.
The judgment of history matters, and it should weigh heavily on all of our minds. Whether Congress does its duty to restrain a lawbreaking President via the impeachment process is largely dependent upon whether those members of Congress are concerned with their standing in the history books. But in an immediate sense, far from the reach of impeachment, it would be a mistake to give any citizen a blanket amnesty to break the law -- even (perhaps especially) the President -- and self-pardoning can be seen as nothing else.
The entirety of Presidential power is derived from an office that exists only to ensure that the law is executed faithfully. There can be no logical leap that gets us to a place where the power to execute the law is simultaneously a power to break the law without consequences.
In other words: Pardon me, but no one can seriously believe that Presidents are allowed to pardon themselves.